This invention relates to flow control devices, such as chokes for hydrocarbon wells,
In a hydrocarbon well, chokes control flow of fluid into production tubing from the well bore or into regions of the well bore from the production tubing. Conventionally, such chokes have been simple on/off devices that merely fully opened or closed the production tubing. Recently, there has been a requirement for variable flow control which has given rise to particular problems. A conventional variable flow control choke is shown in the schematic drawing of FIG. 1a. 
The basic features of this device are an outer sleeve 1 and an inner sleeve 2, each having respective sets 3, 4 of apertures located about their respective circumferences. The outer sleeve 1 may be an integral part of a section of production tubing. The inner sleeve 2 is slidably moveable by means of an actuator (not shown). FIG. 1a shows the location of the sleeves in a xe2x80x9cclosedxe2x80x9d position. FIGS. 1b and 1c show the relative positions of the sleeves in two different xe2x80x9copenxe2x80x9d positionsxe2x80x94partly open and fully open, respectively. The arrows of FIGS. 1b and 1c represent the flow of fluid from the well bore into the production tubing via the apertures 3, 4.
Annular seals 5, 6 and 7 are located between the inner and outer sleeves 2 and 1. These seals separate the annular gap between the inner and outer sleeves into chambers whilst allowing the inner sleeve to move freely. For example, there is an annular chamber 8 between seals 6 and 7, which chamber includes the apertures 3 of the outer sleeve 1.
Activation of the actuator causes the inner sleeve 2 to be moved in the direction of the arrows shown in FIG. 1a. FIG. 1b shows the apparatus of FIG. 1a in a partially open position, wherein the apertures 4 of the inner sleeve encroach on the chamber 8, thereby opening up a flow path. In the fully open position of FIG. 1c, the apertures 4 of the inner sleeve are located entirely within the chamber 8.
Various problems may be encountered with this conventional type of flow control device. For example, as the device begins to enter an xe2x80x9copenxe2x80x9d position, pressure on one side of the seal tends to distort the seal and extrude it in the direction of fluid flow Therefore, in an example shown in FIG. 1b, the seal 6 tends to be extruded into the apertures of the inner sleeve. Should the fluid flow be in the opposite direction (i e. from the production tubing to the well bore), the seal tends to extrude into the annular gap between the sleeves.
Another problem with this type of flow control device is that, at the point of opening, the fluid is flowing very quickly through the apertures, and at high pressure, with the result that the seal 6 can be damaged or dragged into the apertures 4 of the inner sleeve.
The high velocity of the fluid flow in the xe2x80x9cjust openxe2x80x9d position of FIG. 1b can also cause another problem, namely that of erosion of the edges of the apertures, particularly when the fluid is contaminated with solid particles such as sand.
Yet another problem which may be encountered with conventional flow control devices is that the increase of fluid flow rate is not linear with linear movement of the tube and so accurate variable flow control is difficult, especially when low flow rates arc required.
According to the invention there is provided a flow control device, comprising an outer sleeve having an outer sleeve wall and at least one aperture through the outer sleeve wall, an inner sleeve having an inner sleeve wall and at least one aperture through the inner sleeve wall and means for providing relative sliding movement of the sleeves between xe2x80x9copenxe2x80x9d positions allowing variable flow of fluid through the apertures of the sleeves and a xe2x80x9cclosedxe2x80x9d position, so that relative movement of the sleeves produces a variable amount of opening through which fluid can flow, wherein the apertures are arranged so that the change in the coefficient of velocity (CV) of fluid flowing through the apertures caused by a change in the relative displacement of the sleeves is substantially proportional to said change in relative displacement.
One of the sleeves could have a plurality of apertures, the sum of the widths of those apertures at positions along that sleeve increasing substantially exponentially in the direction of fluid flow.
The width of at least one of the apertures could increase in the direction of fluid flow. In this case the width of at least one of the apertures could increase substantially exponentially in the direction of fluid flow.
At least one of the apertures of said one of the sleeves could have a different length from another of those apertures. In this case the plurality of apertures could be grouped in pairs, both apertures of each pair for example having the same length. In the latter case the apertures of each pair could be arranged diametrically opposite each other around the perimeter of said one of the sleeves.
The longest one of the apertures of said one of the sleeves could be narrower than the or each other of those remaining apertures.
The present invention also comprises production tubing including a flow control device according to the invention.
The present invention also comprises a hydrocarbon well including a flow control device according to the invention.
The invention will now be described by way of example, with reference to the accompanying drawings.